The Union Army was divided into many armies throughout the war. They were:
Eastern Theater
- Army of the Potomac, the principal army in the Eastern Theater, commanded by George B. McClellan, Ambrose E. Burnside, Joseph Hooker, and George G. Meade.
- Army of Virginia, the army assembled under John Pope for the Northern Virginia Campaign.
- Army of the James, the army operating on the Virginia Peninsula, 1864-65, commanded by Benjamin Butler and Edward Ord.
- Army of the Shenandoah, the army operating in the Shenandoah Valley, under David Hunter, Philip Sheridan, and Horatio G. Wright.
Western Theater
- Army of the Tennessee, the most famous army in the Western Theater, operating through Kentucky, Tennessee, Mississippi, Georgia, and the Carolinas; commanded by Ulysses S. Grant, William T. Sherman, James B. McPherson, and Oliver O. Howard.
- Army of the Cumberland, the army operating primarily in Tennessee, and later Georgia, commanded by William S. Rosecrans and George Henry Thomas.
- Army of Georgia, operated in the March to the Sea and the Carolinas commanded by Henry W. Slocum.
- Army of the Gulf, the army operating in the region bordering the Gulf of Mexico, commanded by Benjamin Butler, Nathaniel P. Banks, and Edward Canby.
- Army of the Mississippi, a briefly existing army operating on the Mississippi River, in two incarnations-under John Pope and William S. Rosecrans in 1862; under John A. McClernand in 1863.
- Army of the Ohio, the army operating primarily in Kentucky and later Tennessee and Georgia, commanded by Don Carlos Buell, Ambrose E. Burnside, John G. Foster, and John M. Schofield.